order.shippingMethod returns null

[sjelfull]

Description

When placing an order with a custom shipping method, Commerce will try to look up the shipping method from the database when order.shippingMethod is called.

This crashes the order details screen, among other things.

The shipping method is available from getAvailableShippingMethods()

Steps to reproduce

1. Place order with custom shipping method
2. Go to order details screen or call order.shippingMethod

[sjelfull]

Replacing the existing method with this one on craft\commerce\elements\Order fixes this:

/**
 * @return ShippingMethod|null
 */
public function getShippingMethod()
{
    $shippingMethods = Plugin::getInstance()->getShippingMethods()->getAvailableShippingMethods($this);

    return $shippingMethods[$this->shippingMethodHandle] ?? null;
}